The present invention relates to the measurement of fluid flow in general and to a pyroelectric anemometer in particular.
The measurement of fluid flow is fundamental to many scientific and technological areas. In the past mechanical systems which rotate and with the rotation generate a signal which is indicative of flow velocity have been used extensively and are well known embodiments. However, to increase the reliability and decrease the size of anemometers in particular, there has been increasing interest in the development of flow measuring systems that are based upon semiconductor and other solid state phenomena. The hot wire anemometer (HWA) utilizes the heat transfer from a hot wire as an indication of flow velocity. These devices although well developed, remain expensive and difficult to use.
Many of the difficulties with respect to the HWA have not been overcome even with the most recent solid state devices such as the thermistor anemometer, the transistor anemometer, and the integrated silicon anemometer. The first two types of anemometers are discussed at the February 1980 issue of IEEE Spectrum in an article entitled "Microprocessors get Integrated Sensors" by Middelhoek, Angell, and Noorlag at page 42. In a thermistor anemometer, air flowing over a resistor cools the resistor and changes its resistance which resistance is an indication of flow velocity. In the transistor type anemometer, a heating transistor heats up a small integrated circuit chip over which the fluid whose velocity is to be measured is flowing. Upstream and downstream transistors are, respectively, cooled and heated by the flowing gas in combination with the heating transistor. The difference in temperature between the two transistors causes a variation in their gain and thus their output signal and are indicative of flow velocity. Unfortunately, the output signal from the transistor anemometer is a non-linear function of the flow velocity unless a number of conditions are met and substantial signal processing is utilized.
A further type of flow measuring device is disclosed in U.S. Pat. No. 3,519,924 to Burton in which a piezoelectric oscillator is heated by a heating element and the heat transfer from the oscillator to the fluid is a function of the fluid velocity and can be measured by comparing the frequency of oscillation of the test oscillator with an isolated reference oscillator.